In oil and gas drilling rigs and in workover units, there is frequently a need for a man to work in the derrick. In large drilling rigs, often the derrick man will be working at least 90 foot above the rig floor. In case the well blows out or the rig catches on fire, the derrick man would be trapped unless some means is provided for him to safely reach the ground. Climbing down the derrick itself would not be possible ordinarily in case of a fire or blow out.
Drilling rigs have long used safety lines for the derrick man. The safety line, or "Geronimo" line, inclines from a point in the derrick near the derrick man's station, or "monkey board", to a point on the ground some distance away. There are various types of slings with hand brakes with which the derrick man can slide down the line. However, many of these hand brakes and slings are difficult to operate and dangerous. The derrick man is likely to injure himself, even if the sling and brake had been properly maintained, which is often not the case.
In addition to emergencies, it is not infrequent for injuries to occur as a result of the derrick man climbing to and from the monkey board. Normally, the only means by which the derrick man can reach the monkey board and return to the derrick floor is by an uncaged ladder. This is particularly dangerous during wet or icy weather, and because of water and drilling mud often lying on the rig floor. Various patents exist that disclose proposals to remedy some of the problems, however these have not been widely adopted.